A student places a hot drink in a polystyrene cup and the same volume of equally hot drink in a thin metal cup. After ten minutes, the drink in the metal cup has cooled to a much lower temperature than the drink in the polystyrene cup. Explain why the drink in the metal cup cools more quickly.

WJEC A-Level Physics (Wales) — 3.4 Thermal physics · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Polystyrene is a thermal insulator. Metal is a good thermal conductor.

Model answer (5 marks)

Heat is transferred from the hot drink to the cup. The metal cup is a good thermal conductor, so the thermal energy passes through it quickly to the outer surface, where it is lost to the surrounding air. The polystyrene cup is a thermal insulator, so the energy passes through it very slowly, keeping the drink hotter for longer.

Examiner tips

  • Use the word ‘heat’ and ‘thermal energy’ early; mention conduction and transfer to air.
  • Show the sequence: drink → cup → surroundings, emphasising metal’s high conductivity and polystyrene’s low conductivity.
  • Keep the answer concise – 5 marks can be earned with 3–4 short points.
  • Use UK spelling (e.g. ‘insulator’).

Common mistakes

  • Confusing the cup material with the drink’s temperature change; forgetting to mention transfer to the surroundings.
  • Using vague terms like ‘fast’ without linking to conductivity.
  • Writing a long paragraph instead of clear points.

Mark scheme (5 marks)

  1. Heat/thermal energy is transferred from the hot drink to the cup
  2. Metal is a good thermal conductor so it allows thermal energy to pass through it easily / quickly
  3. Thermal energy is then transferred from the outer surface of the metal cup to the surroundings / air
  4. Polystyrene is a thermal insulator so it does not allow thermal energy to pass through it easily
  5. So the drink in the polystyrene cup loses thermal energy more slowly / at a lower rate, keeping it hotter for longer

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